Introduces `WP_Site_Health::has_late_cron()` for late wp-cron jobs and extends the time allowance before a job is considered missed.
In a standard configuration using loopback requests, a job is considered late once past due and missed over five minutes past due.
Late and missed time frames are extended if `DISABLE_WP_CRON` is defined as `true` to allow for crontab tasks running less frequently. A job is considered late once it's 15 minutes past due and missed over one hour past due.
A file for site health unit tests has been introduced with tests for cron in critical, late and missed states.
Props rockfire, afragen, peterwilsoncc.
Fixes #47223.
git-svn-id: https://develop.svn.wordpress.org/trunk@45801 602fd350-edb4-49c9-b593-d223f7449a82